In four-cycle gasoline engines (internal combustion engines) included in automobiles or the like, electronically controlled fuel injection devices have been used in order to deal with emission control, improve fuel economy, or improve driving performance.
For an engine in which fuel injection is electronically controlled, an amount of air flowing into a cylinder needs to be estimated in order to determine an amount of fuel injected from an injector so as to obtain air-fuel mixture with a predetermined air-fuel ratio. As a method for estimating an inflow air amount into a cylinder of an engine, a method (a speed density method) is known for estimating an inflow air amount from intake pipe pressure (pressure within an intake pipe) and a rotational speed of an engine.
For an engine in which a fuel injection amount is arithmetically operated with respect to an intake air amount estimated from intake pipe pressure and a rotational speed, an air-fuel ratio goes lean by detection delay of intake pressure when a driver abruptly opens a throttle in an attempt to accelerate a vehicle, thereby causing deterioration of exhaust gas components or degradation of driving performance of the engine. In order to avoid such problems, acceleration increase correction for increasing a fuel injection amount according to an amount of change in throttle opening degree needs to be performed when the throttle is abruptly operated to accelerate the engine, thereby preventing the air-fuel ratio from going lean.
Generally, a throttle position sensor that detects a position of a throttle valve is used as a method for detecting an operation amount of a throttle. Using the throttle position sensor, however, inevitably increases costs of an engine. In order to reduce costs, it is required that an acceleration operation of the engine is detected without using the throttle position sensor to perform acceleration increase correction.
As a method for detecting an acceleration operation of an engine without using a throttle position sensor, Japanese Patent Application Laid-Open Publication No. 2002-242749 proposes a method for monitoring intake pipe pressure of an engine and detecting an acceleration state of the engine when the intake pipe pressure represents a predetermined change.
In a method described in Japanese Patent Application Laid-Open Publication No. 2002-242749, a plurality of preset crank angle positions are determined as sample positions, intake pipe pressure is sampled at each sample position, and intake pipe pressure newly sampled at each sample position is compared with intake pipe pressure sampled at the corresponding sample position one combustion cycle before. Then, when the intake pipe pressure newly sampled is a predetermined value or more higher than the intake pipe pressure sampled one combustion cycle before, an acceleration state of the engine is determined.
In the acceleration of the engine, the intake pipe pressure is increased by opening a throttle valve. Thus, the intake pipe pressure sampled at each sample position is compared with the intake pipe pressure sampled at the corresponding sample position one combustion cycle before to detect that the intake pipe pressure newly sampled is a predetermined level or more higher than the intake pipe pressure sampled one combustion cycle before, thereby allowing the acceleration state of the engine to be detected. The degree of acceleration can be determined by, for example, checking a rate of change with time in a difference between the intake pipe pressure newly sampled and the intake pipe pressure sampled at the corresponding sample position one combustion cycle before.
With the acceleration detection method described in Japanese Patent Application Laid-Open Publication No. 2002-242749, the acceleration state of the engine can be detected without using a throttle position sensor. However, determining the plurality of crank angle positions of the engine as the sample positions and detecting the acceleration based on the intake pipe pressure sampled at each sample position causes the following problems.
The intake pipe pressure quickly decreases in an intake stroke, and represents a minimum value at an end of the intake stroke or a start of a compression stroke. The intake pipe pressure represents the minimum value, and then gradually increases until immediately before the next intake stroke. The degree of increase in a process for the intake pipe pressure to increase is controlled by a time constant determined by an opening area of a throttle valve (a throttle opening area) and a capacity of an intake pipe between the throttle valve and an intake valve (a capacity of the intake pipe downstream of the throttle valve). The intake valve is closed in the process for the intake pipe pressure to increase, and thus the intake pipe pressure is independent of movement of a piston (a crank angle).
When a throttle opening degree is small in the process for the intake pipe pressure to increase, a low flow rate of air passes through an opening of the throttle valve and the intake pipe pressure thus increases slowly, while when the throttle opening degree is large, a high flow rate of air passes through the opening of the throttle valve and the intake pipe pressure thus increases quickly. Thus, in a period when the intake pipe pressure increases after the intake stroke is finished, the intake valve is closed and thus independent of the crank angle. Also, the capacity of the intake pipe is fixed, and thus the intake pipe pressure is determined by the opening area of the throttle valve and elapsed time.
In the proposed acceleration and deceleration detection method, the preset crank angle positions are determined as the sample positions to sample the intake pipe pressure even in the period when the intake pipe pressure increases after the intake stroke is finished. Thus, in a state where a rotational speed of the engine changes (a transient state), the intake pipe pressure sampled at each sample position and the intake pipe pressure sampled at the corresponding sample position one combustion cycle before do not have correspondence as a subject of comparison, and the state of acceleration of the engine cannot be accurately detected.
The change in the intake pipe pressure after the intake valve is closed is determined by the opening area of the throttle valve and the elapsed time, and thus it is supposed that information on the opening area of the throttle valve is obtained from the change in the intake pipe pressure and used for various types of control of the engine. If the information on the opening area of the throttle valve can be obtained without using a throttle sensor, costs of control devices can be reduced when various types of control amounts are controlled with respect to the opening area of the throttle valve in addition to fuel injection control.
As described above, the acceleration of the engine has been detected from the change in the intake pipe pressure, but the information on the opening area of the throttle valve has not been obtained from the intake pipe pressure.